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We present results from simultaneous optical (SALT) and X-ray (Swift andINTEGRAL) observations of GS 1354-64/BW Cir during the 2015 hard stateoutburst. During the rising phase, optical/X-ray time series show a stronganti-correlation with X-ray photons lagging optical. Optical and X-ray powerspectra show quasi-periodic oscillations at a frequency of ~18 mHz with aconfidence level of at least 99%. Simultaneous fitting of Swift/XRT andINTEGRAL spectra in the range 0.5-1000 keV shows non-thermal, power-lawdominated (> 90%) spectra with a hard power-law index of 1.48 +/- 0.03, innerdisc temperature of 0.12 +/- 0.01 keV and inner disc radius of ~3000 km. Allevidence is consistent with cyclo-synchrotron radiation in a non-thermal, hotelectron cloud extending to ~100 Schwarzschild radii being a major physicalprocess for the origin of optical photons. At outburst peak about one monthlater, when the X-ray flux rises and the optical drops, the apparent featuresin the optical/X-ray correlation vanish and the optical auto correlationwidens. Although ~0.19 Hz QPO is observed from the X-ray power spectra, theoptical variability is dominated by the broadband noise, and the inner disctemperature increases. These results support a change in the dominant opticalemission source between outburst rise and peak, consistent with a weakening ofhot flow as the disc moves in.

monthly notices of the royal astronomical society

Simultaneous Optical/X-ray study of GS 1354-64 (=BW Cir) during hard  outburst: evidence for optical cyclo-synchrotron emission from the hot  accretion flow